1. Technical Field
This disclosure relates to differential mobility analyzers and more particularly to differential mobility analyzers that yield a mobility-equivalent spherical diameter.
2. Description of Related Art
Nanomaterials are widely applied and studied in medicine, electronics, biomaterials and environmental science. Effective measurement and accurate characterization of nanomaterials play a critical role in the development of nanotechnology. It is well-known that many of the properties of particles are size dependent. Moreover, for aspherical structures such as nanorods, nanowires, the properties are also greatly influenced by their shapes.
For example gold nanorods are useful for the formation of many functional composite materials due to their special light scattering and absorption properties (Ni et al. 2008; Alkilany et al. 2012). Non-spherical particles also have important effects on environment and human health. Soot aggregates produced by combustion are highly non-spherical ramified structures with non-integer fractal dimensions. The common feature of all these materials is that they are non-spherical and thus cannot be dimensionally characterized by just one length scale. To obtain size and shape information of nanoparticles, microscopy techniques, such as transmission or scanning electron microscopy (TEM/SEM), are traditionally applied. However, in these off-line methods, good sampling methods and time-consuming operations are needed for a precise distribution measurement. It is also reported that the sampling and imaging process itself may cause coalescence of small clusters (Schmid and Chi 1998).
One of the major challenges in particle online measurement is to extend the dimensionality measurement beyond the assumption of spherical symmetry. For a nonspherical particle, a differential mobility analyzer (DMA) measurement yields a mobility-equivalent spherical diameter, but provides no information about the degree of sphericity.
The differential mobility analyzer (DMA) is the gold-standard on-line measurement method for obtaining a complete electrical-mobility-size distribution of nanoparticles in the aerosol phase (Flagan 2008). For a spherical particle, the electrical mobility diameter is equivalent to its geometric diameter. However, if the particle is non-spherical, the resulting electrical mobility diameter is that diameter for a sphere with the same mobility as the analyte particle. For example Song et al. (Song et al. 2005) investigated the relationship between the electrical mobility size and particles shape, by changing the particle shape from nanorod to sphere by heating the particles from 25° C. to 800° C., and showed that the mobility diameters decreased from 55 nm to 25 nm. Since the mobility size measured in the DMA depends on the drag force on the particles, thus for a non-spherical particle, mobility necessarily depends on orientation with respect to the applied electric field (Kousaka et al. 1996; Zelenyuk and Imre 2007; Kim et al. 2007; Li et al. 2012; Li et al. 2013). In principle then, an orientation dependent mobility measurement should yield some information on particle shape.
Kousaka et al. (1996) measured the dynamic shape factor for doublets of uniform spheres (Polystyrene latex particles; PSL) in the transition regime and pointed out that the orientation of doublets is a function of electric field in the DMA and the size of doublets. Zelenyuk and Imre (2007) applied this idea to more aspherical particles and showed that the dependence of electrical mobility size on electric field can be applied to separate particles based on their shape. Kim et al. (2007) measured the length of carbon nanotubes considering a scalar expression of drag force.